The meaning of real, user, and sys in output of Li

Question

This question already has an answer here:

• What do 'real', 'user' and 'sys' mean in the output of time(1)? 5 answers

$ time ./Test 

real    0m2.906s
user    0m2.887s
sys     0m0.017s

Here is the program code:

#include <iostream>
#include <map>

void func_a() {
    std::map<int, int> m;
    for (unsigned int i = 0; i < 10000; i++) {
        m.insert(std::pair<int, int>(i, i));
    }
}

void func_b() {
    std::map<int, int> m;
    for (unsigned int i = 0; i < 1000000; i++) {
        m.insert(std::pair<int, int>(i, i));
    }
}

int main() {
    func_a();
    func_b();
    return 0;
}

Answer 1

If you take a look at the manpage (man time), it states:

The time command runs the specified program command with the given arguments. When command finishes, time writes a message to standard output giving timing statistics about this program run. These statistics consist of (i) the elapsed real time between invocation and termination, (ii) the user CPU time (the sum of the tms_utime and tms_cutime values in a struct tms as returned by times(2)), and (iii) the system CPU time (the sum of the tms_stime and tms_cstime values in a struct tms as returned by times(2)).

Basically though, the user time is how long your program was running on the CPU, and the sys time was how long your program was waiting for the operating system to perform tasks for it. If you're interested in benchmarking, user + sys is a good time to use. real can be affected by other running processes, and is more inconsistent.